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Income Taxes - Summary of Consolidated Federal Income Tax Return (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of consolidated Federal income tax return
Current city income tax provision	$ 935,612dhil_CurrentLocalTaxExpenseBenefit	$ 952,957dhil_CurrentLocalTaxExpenseBenefit	$ 406,814dhil_CurrentLocalTaxExpenseBenefit
Current state income tax provision	429,147dhil_CurrentStateTaxExpenseBenefit	268,920dhil_CurrentStateTaxExpenseBenefit	122,704dhil_CurrentStateTaxExpenseBenefit
Current federal income tax provision	15,015,813us-gaap_CurrentFederalTaxExpenseBenefit	17,866,911us-gaap_CurrentFederalTaxExpenseBenefit	8,990,777us-gaap_CurrentFederalTaxExpenseBenefit
Deferred federal income tax benefit	2,404,433	(5,611,451)	(368,572)
Provision for income taxes	$ 18,785,005us-gaap_IncomeTaxExpenseBenefit	$ 13,477,337us-gaap_IncomeTaxExpenseBenefit	$ 9,151,723us-gaap_IncomeTaxExpenseBenefit

Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred federal income tax expense (benefit) pertaining to income (loss) from continuing operations.